Spatial light modulators such as liquid crystal displays (LCDs) and digital mirror devices (DMDs) (as available from Texas Instruments, for example) are used to modulate incident light into a spatial pattern to form a light image corresponding to an electrical or optical input. DMDs have been successfully incorporated into video projectors and printers, for example. See U.S. Pat. No. 5,535,047 incorporated herein by this reference.
In the field of microscopes, it is often desirable to create a mask pattern to vary the illumination or viewing properties of the microscope using techniques such as transmissive illumination, incident illumination, dark field illumination, bright field illumination, oblique illumination, differentially shaded illumination, phase contrast illumination, differential polarization illumination, and the like. In one example, a technique known as fluorescence recovery after photobleaching (FRAP) involves labeling specific proteins within a living cell with fluorescent dyes and then selected areas are irreversibly photobleached by an intense flash of light and the diffusional mobility of the protein is measured by measuring the fluorescence recovery through the exchange of bleached for non-bleached protein. In this example, a small area of the slide containing the cells is targeted and measured comparatively to the surrounding structure. In this procedure, the ability to control the spatial distribution of the illumination of the microscope for targeting and measurement is critical.
In the past and even today these techniques were accomplished using mechanical pinholes or irises to form the mask pattern image. See U.S. Pat. No. 4,561,731 also incorporated herein by this reference.
After the advent of spatial light modulators such as LCDs and DMDs, however, those skilled in the art soon began proposing these types of modulators in microscope systems instead of pinholes or irises to form mask patterns. A DMD is shown in U.S. Pat. No. 5,535,047 incorporated herein by this reference.
Surprisingly, however, the art is currently limited a) to specially configured microscopes employing LCDs and DMDs, (see, for example, U.S. Pat. Nos. 5,923,036; 5,587,832; and 5,923,466 incorporated herein by this reference) or, alternatively, b) to a microscope coupled to a complete video projector—the video projector itself incorporating, inter alia, an LCD or DMD, a light source, and the associated driver. See U.S. Pat. No. 6,243,197 incorporated herein by this reference.
The drawbacks to such configurations are many. Specially designed microscopes are expensive and render obsolete the user's existing microscopes. Video projector type illuminating devices coupled to an existing microscope, on the other hand, results in an unduly complex, bulky, and expensive design, and, moreover, results in flickers and restrictions on the range of wavelengths which can be used in the microscope due to, inter alia, both the video projector design and the video input signal which operates the video projector.
For example, the use of a commercially available video projector with a DMD controlled by a computer graphics card for illumination in a microscope (U.S. Pat. No. 6,243,197) has significant drawbacks. Since video projectors typically use a rotating color wheel between the light source and the DMD, the micromirrors must be synchronized to the primary color segments of the spinning wheel. This type of illumination inherently ‘flickers’ and, although acceptable for overhead projections, is not adequate for scientific microscopy. The temporal switching inherent in the method of pulse-width modulation (PWM) used to vary intensity levels in a projector system, combined with the electrical noise of the computer graphic card interface, may result in unacceptable flicker in a microscope system. Furthermore, the spectral nature of the light source and optical coatings used in video projectors would restrict microscope studies to a wavelength range narrower than usual.
What is needed is an optical head which can be easily coupled to a wide variety of existing microscopes and employing DMD or other spatial light modulation technology to digitally generate mask patterns in real time and without the necessity of the other components and the limitations associated with video projectors. Such a spatial light modulator apparatus is useful, for example, when carrying out FRAP and other techniques and procedures.